An accelerometer is one of the primary sensors used in on-board automotive safety control systems and navigational systems, particularly crash sensing systems. Examples of such automotive applications include anti-lock braking systems, active suspension systems, supplemental inflatable restraint systems such as air bags, and seat belt lock-up systems. An accelerometer is a device which measures acceleration or, more accurately, accelerometers measure the force that is exerted by a proof mass as a result of a change in the velocity of the mass.
In a micromachined accelerometer employing oppositely disposed piezoresistive microbridges supporting the proof mass, acceleration causes compressive or tensile load on portions of the microbridges. In turn, the resulting compressive or tensile loads change electrical resistance of piezoresistors in the microbridges. This change in resistance can be sensed to determine the magnitude of the acceleration or component of the acceleration along an axis normal to the plane of the bridges. An accelerometer circuit connected to the piezoresistors processes signals affected by the resistance change to produce an output voltage which is proportional to the acceleration along that axis.
It is desirable to package the accelerometer and the accelerometer circuit together and interconnect them by electrical leads. The circuit typically comprises an alumina substrate supporting a hybrid circuit including thick film resistors which determine the circuit parameters. During manufacture, the circuit parameters are initially set to obtain an approximately correct signal for a given acceleration; however, the circuit is temperature sensitive. A functional calibration is made at two different temperatures by applying a test acceleration and trimming resistors on the accelerometer circuit to yield the desired output at each temperature. In the past, this calibration has been accomplished at each temperature by accelerating the package at a specified rate and measuring the output of the circuit at a test station, transferring the package to a trimming station, laser trimming the resistor by a calculated amount to hopefully correct any error in the output, and repeating the acceleration and measurement to verify that the trimming was sufficient. If not, further trimming and testing ensue. The process of test, transfer, trim, transfer, test, etc., is time consuming and expensive.
In the past, it has been known to functionally calibrate other types of electronic devices, such as pressure sensors, by laser trimming the resistors at the same time the pressure or other parameter is applied so that the testing and verification are coincident, that is, for a given temperature, the resistor trimming progresses until the desired value is attained and no further verification is required, and the process is repeated at another temperature. In the case of accelerometers, such simplicity of calibration has not heretofore been realized due to the difficulty of simultaneously mechanically exciting the package and trimming the resistors.